1. Field of the Invention
This invention relates to an optical transmission system for analog signals, and in particular to a directly modulated solid-state laser. More particularly, the invention relates to the use of an electronic circuit for providing a linear output from an amplitude modulated transmission device such as a semiconductor laser which has an output distorted from its input due to inherent nonlinearity. The distortion of the nonlinear device is compensated by applying a predistorted signal to the input of the nonlinear device. The predistortion is chosen such that the distortion of the nonlinear device restores the undistorted signal, enabling transmission over long lengths of dispersive fiber optic media at 1550 nm.
2. Description of the Related Art
Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as voice and video signals, on optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation may suffer from noise and nonlinearity of the optical source.
For that reason, direct modulation techniques have been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links, the low loss of the link requires that externally modulated 1550 nm lasers be used, but such external modulation techniques are complex and expensive. The present invention is therefore addressed to the problem of providing a simple and low cost system for direct modulation of a laser at 1550 nm so that the analog optical output can be used in metro and long haul optical networks using analog transmission.
Direct modulation of lasers at 1550 nm is known for use in digital optical transmission systems such as dense wavelength division multiplexing (DWDM) systems. See, for example, Kartalopoulos, DWDM Networks, Devices, and Technology (IEEE Press, 2002).
One of the difficulties in designing an analog system at 1550 nm is that suitable low chirp lasers for use at 1550 nm are not known in the prior art. One type of low chirp laser is the external cavity laser, which is used in digital optical transmission systems, and is a commercially available product.
In addition to the low chirp characteristics required for an analog optical transmission system at 1550 nm, the system must be highly linear. Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission, which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial analog systems, such as broadcast TV transmission, CATV, interactive TV, and video telephone transmission.
Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages. Most applications discussed above have bandwidths, which are too large for many practical implementations. Feedforward techniques require complex system components such as optical power combiners and multiple optical sources. Quasi-optical feedforward techniques suffer from similar complexity problems and further require extremely well matched parts.
One method employed in the past to reduce distortion inherent in lasers or other nonlinear devices has been the use of predistortion circuits. In this technique, a modulation signal is combined with a signal equal in magnitude to the distortion inherent in the nonlinear device but opposite in sign. When the nonlinear device modulates the combined signal, the device's inherent distortion is canceled by the combined signal's predistortion and only the linear part of the source signal is transmitted. This predistortion signal is usually in the form of additive and subtractive combinations of the input fundamental frequencies, as these intermodulation products constitute the most fertile source of distortion in analog signal transmission. In the distribution of AM signals for cable television, for example, there are often as many as 100+ frequencies on a particular band and numerous opportunities for second order and third order intermodulation products of those frequencies.
These predistortion techniques have been used in current 1310 nm optical transmitters and are exemplified by U.S. Pat. No. 6,288,814, which is hereby incorporated by reference.
Some of the early predistortion techniques generally divide an input signal into two or more electrical paths and generate predistortion on one or more of the paths resembling the distortion inherent in the nonlinear transmitting device. The generated predistortion is the inverse of the nonlinear device's inherent distortion and serves to cancel the effect of the device's inherent distortion when recombined with the input signal.
Attenuation can be used to match the magnitude of the predistortion to the magnitude of the device's inherent distortion characteristics before the signals are recombined and sent to the nonlinear device for modulation. However, the method suffers from crudeness because nonlinear devices frequently have amplitude and phase distortion characteristics dependent on the frequency of the modulating signal. More recent techniques provide means for compensating for these frequency-dependent nonlinearities.
Neglecting to correct for the frequency dependence of the distortion leads to a result that may be quite tolerable for many systems and for signals with relatively narrow bandwidth. However, they become particularly troublesome when converting an electrical TV signal to an optical signal for cable transmission. Such signals for cable TV may have forty or more input frequencies, all of which need to have high quality amplitude modulated signals. The transmission devices for such signal must have an exceptionally high degree of linearity.
Advanced multi-path predistortion circuits are flexible and highly effective for linearizing output of a wide range of nonlinear devices. One such multi-path predistortion circuit is disclosed in U.S. Pat. No. 4,992,754, issued to Blauvelt et al. The circuit is capable of generating frequency specific distortion products for compensating frequency-dependent nonlinearities, and is useful for applications requiring an exceptionally high degree of linearity, such as, for example, CATV applications.
Although multi-path predistortion circuits can be used in a broad variety of applications, the design of these circuits is relatively complex. This complexity manifests itself in circuits that are often too expensive for applications needing only a modest degree of linearization. One skilled in the art would appreciate a low-cost circuit of relatively simple design for limited application, particularly if such a circuit were fabricated from existing low-cost components commonly used in signal transmission applications.
Circuits as described here could produce frequency dependent third-order distortion. Simple third-order distortion, such as that produced by an ideal diode, has the property that the distortion is real and independent of frequency. Many non-linear transmitters or amplifiers, however, contain reactive elements such as inductances, capacitances or delays, which cause the device to produce distortion depending on the input and output frequencies and the distortion frequencies. Nazarathy, U.S. Pat. No. 5,161,044, discloses a circuit in FIG. 15 of that patent which produces essentially real, frequency-independent predistortion. The capacitors and inductors in Nazarathy are added for biasing purposes and to block the DC and AC currents. However, the circuit disclosed by Nazarathy may not have the right phase or frequency dependence for each set of input frequencies, to be substantially the same in magnitude and opposite in sign to the distortion produced by the non-linear device.
The present invention accordingly is addressed to overcoming these and other difficulties found in designing and operating 1550 nm laser analog optical transmission systems.